FSH is a key regulator of testis function, required for the establishment of full complements of Sertoli and germ cells during postnatal testis development and for the maintenance of spermatogenesis in the adult. FSH plays an important role in germ cell survival rather than proliferation, in the window between 14 and 18 days of testicular development, which coincides with the cessation of Sertoli cell proliferation and the onset of germ cell meiosis during the first wave of spermatogenesis. This study aimed to identify the pathway(s) of apoptosis regulated by changes in FSH levels in 14 - to 18-day-old rats, using a model of in vivo FSH suppression by passive immunoneutralization with a rat anti-FSH antibody. Apoptotic pathways were identified by immunohistochemistry using pathway-specific proteins as markers of the intrinsic (activated caspase 9) and extrinsic (activated caspase 8) pathways, followed by quantification of cell numbers using stereological techniques. In addition, RT-PCR was used to assess the expression of pathway-specific genes. We previously reported a 2.5-fold increase in spermatogonial apoptosis in these samples after 4 days of FSH suppression, and now show that this increase correlates with a 9.8-fold (P<0.001) increase in the frequency of caspase 9-positive spermatogonia in the absence of caspase 8 immunoreactivity. By contrast, spermatocytes exhibited both increased caspase 9 (7.5-fold; P<0.001) and caspase 8 (5.7 fold; P<0.001) immunoreactivities after 4 days of FSH suppression. No significant change in the transcription levels of candidate genes required for either pathway was detected. This study demonstrates that, in the seminiferous tubules, FSH suppression induces spermatogonial apoptosis predominantly via the intrinsic pathway, while spermatocyte apoptosis occurs via both the intrinsic and extrinsic pathways.

The critical influence of follicle stimulating hormone (FSH) on male fertility relates both to its impact on Sertoli cell proliferation in perinatal life and to its influence on the synthesis of Sertoli cell-derived products essential for germ cell survival and function in the developing adult testis. The nature and timing of this shift of germ cells to their reliance on specific Sertoli cell-derived products are not defined. Based on existing data, it is apparent that the dominant function of FSH shifts between 9 and 18 day postpartum (dpp) during the first wave of spermatogenesis from driving Sertoli cell proliferation to support germ cells. To enable comprehensive analysis of the impact of acute in vivo FSH suppression on Sertoli and germ cell development, FSH was selectively suppressed in Sprague–Dawley rats by passive immunisation for 2 days and/or 4 days prior to testis collection at 3, 9 and 18 dpp. The 3 dpp samples displayed no measurable changes, while 4 days of FSH suppression decreased Sertoli cell proliferation and numbers in 9 dpp, but not 18 dpp, animals. In contrast, germ cell numbers were unaffected at 9 dpp but decreased at 18 dpp following FSH suppression, with a corresponding increase in germ cell apoptosis measured at 18 dpp. Sixty transcripts were measured as changed at 18 dpp in response to 4 days of FSH suppression, as assessed using Affymetrix microarrays. Some of these are known as Sertoli cell-derived FSH-responsive genes (e.g. StAR, cathepsin L, insulin-like growth factor binding protein-3), while others encode proteins involved in cell cycle and survival regulation (e.g. cyclin D1, scavenger receptor class B 1). These data demonstrate that FSH differentially affects Sertoli and germ cells in an age-dependent manner in vivo, promoting Sertoli cell mitosis at day 9, and supporting germ cell viability at day 18. This model has enabled identification of candidate genes that contribute to the FSH-mediated pathway by which Sertoli cells support germ cells.